1. Field of the Invention
The present invention relates in general to an apparatus for, and the processing of, semiconductor substrates. In particular, the invention relates to the etching of anti-reflective coatings during substrate processing.
2. Related Art
The critical dimensions (CD's) and geometries of semiconductor devices have decreased dramatically in size since they were first introduced several decades ago. Although, currently, most semiconductor devices are fabricated with feature sizes of about 0.5 microns, it is desirable to produce semiconductor devices, for example, semiconductor integrated circuit chips, with smaller feature sizes, such as 0.35 microns and lower.
One important micro geometry of such semiconductor devices includes the formation of a patterned thin film on a base substrate of the device by chemical reaction of gases. When patterning the thin films, it is desirable that fluctuations in line width and other critical dimensions be minimized. Errors in these dimensions can result in variations in device characteristics or open-/short-circuited devices, thereby adversely affecting device yield. Thus, as feature sizes decrease, structures must be fabricated with greater accuracy. As a result, some manufacturers now require that variations in the dimensional accuracy of patterning operations be held to within 5% of the dimensions specified by the designer.
Some substrate processing systems employ photolithographic techniques to pattern layers. These techniques employ photoresist or other light-sensitive material deposited on a wafer. A photomask having transparent and opaque regions embodying the desired pattern is positioned over the photoresist. When the mask is exposed to light, the transparent portions allow exposure of the photoresist in the transparent regions, but not in the regions where the mask is opaque. The light causes a chemical reaction in the exposed portions of the photoresist (e.g., photosolubilization or polymerization). A suitable chemical, chemical vapor or ion bombardment process is then used to selectively attack either the reacted or unreacted portions of the photoresist. With the remaining photoresist pattern acting as a mask, the underlying layer may then undergo further processing. For example, the layer may be doped or etched, or other processing can be performed.
Some photolithographic techniques often involve the use of equipment known as steppers, which are used to mask and expose photoresist layers. Steppers often use monochromatic (single-wavelength) light, enabling them to produce the detailed patterns required in the fabrication of fine geometry devices. However, as a substrate is processed, the topology of the substrate's upper surface becomes progressively less planar. This uneven topology can cause reflection and refraction of the monochromatic light, resulting in exposure of some of the photoresist beneath the opaque portions of the mask. As a result, this uneven surface topology can alter the mask pattern transferred to the photoresist layer, thereby altering the desired dimensions of the structures subsequently fabricated.
One phenomenon resulting from these reflections is standing waves. When a photoresist layer is deposited on a reflective underlying layer and exposed to monochromatic radiation (e.g., deep ultraviolet (UV) light), standing waves may be produced within the photoresist layer. In such a situation, the reflected light interferes with the incident light and causes a periodic variation in light intensity within the photoresist layer in the vertical direction. Standing-wave effects are usually more pronounced at the deep UV wavelengths used in modern steppers than at longer wavelengths because surfaces of materials such as oxide, nitride, and polysilicon are more reflective at deep UV wavelengths. The existence of standing waves in the photoresist layer during exposure causes roughness in the vertical walls formed when sections of the photoresist layer are removed during patterning, which translates into variations in linewidths, spacing and other critical dimensions.
There are several techniques currently used to help reduce and/or eliminate the standing waves and problems associated with these waves, while achieving required dimensional accuracies. The use of an anti-reflective coating (ARC) is one such technique. An ARC's optical characteristics are such that reflections occurring at inter-layer interfaces are minimized. The ARC's absorptive index is such that the amount of monochromatic light transmitted (in either direction) is minimized, thus attenuating both transmitted incident light and reflections thereof. The ARC's refractive and absorptive indices are fixed at values that cause any reflections which might still occur to be canceled.
One type of ARC is a titanium nitride anti-reflective coating (TiN ARC). TiN ARC's are typically used with semiconductor substrates that have conductive features. Conductive features are used to electrically connect devices formed on the semiconductor substrates. The conductive features typically have a bottom barrier layer, an electrically conductive metal-containing layer, such as an aluminum alloy, and a top titanium nitride anti-reflective coating. Although titanium nitride was once deemed to be a satisfactory anti-reflective coating, it has been recently determined that it is not a relatively good anti-reflective coating. Consequently, another anti-reflective layer, such as an organic anti-reflective coating (OARC) or a bottom anti-reflective coating (BARC) is usually spun on top of the TiN ARC.
The substrate with the additional OARC layer is then processed by reactive ion etch processing to selectively etch portions of the substrate. Etching comprises introducing a selected process gas into an etching chamber and producing a plasma from the process gas. The plasma selectively etches the substrate and creates volatile etch by product compounds which are removed from the etching chamber. The process gas typically used for etching the OARC and TiN ARC is a mixture of gases such as, for example, Cl.sub.2 with N.sub.2, Ar or BCl.sub.3.
During the etch process, several factors must be considered, such as CD control, etch rate non-uniformity, photoresist loss and etch selectivity. Good CD control and etch rate uniformity allows the fabrication of semiconductor devices with smaller feature sizes. In addition, with regard to photoresist loss and etch selectivity, a material's reactivity with respect to another material with regard to a given etchant is known as the material's etch selectivity. Etch selectivity is usually denoted by a ratio of the etch rate of the material to be removed to that of the other material. A high etch selectivity is therefore often desirable because, ideally, an etchant should selectively etch only the intended areas of the layer being etched and not erode other structures which may already exist on the substrate being processed. In other words, a material with high etch selectivity substantially resists unintended etching during the intended etching of another material.
For example, high etch selectivity of a first layer with respect to a second overlying layer is desirable when different patterns are to be etched into the first and second layers. High etch selectivity is desirable in such situations because the underlying layer will not be significantly eroded in areas where the second layer is completely etched away if the first layer's etch selectivity is low. The etching operation removes not only the intended regions of the second layer, but also portions of the first layer underlying those regions. While a small amount of the first layer is normally removed in such situations, extremely low etch selectivity may permit substantial etching of the first layer.
However, the current combination of process gases used to etch the OARC and TiN ARC produces relatively poor CD control and non-uniform etch rates. Also, typical etch selectivities are relatively low and photoresist loss is relatively high. Excessive etching of the photoresist layer can cause excessive deposition of polymeric resist etchant byproducts on the substrate and on the walls of the etching apparatus. Excessive quantities of such deposits are difficult to remove. Thus, typical etch processes used to etch the OARC and TiN ARC produce inadequate results due to the increased photoresist loss and poor CD control, etch rate uniformity and etch selectivity. As a result, currently, the addition of an OARC to a TiN ARC on a semiconductor substrate makes the etch process more difficult and complicated.
Therefore, what is needed is an etch system for effectively etching the OARC and TiN ARC layers that has good CD control and limits etch rate non-uniformity. What is also needed is a system that avoids unwanted etching of layers underlying the layer being patterned. What is also needed is a system for creating such a layer using a minimal number of processing steps. What is additionally needed is an accurate etch process. What is additionally needed is an etching system that provides high etch selectivity. What is further needed is an etching system that is amenable to mass production of circuit chips in conventional etching apparatus.
Whatever the merits of the above mentioned systems and methods, they do not achieve the benefits of the present invention.